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D. Iskander, M. J. Collins, T. Buehren, B. Davis; The Applicability of Image Quality Metrics in the Estimation of the Sphero-Cylindrical Refraction From Wavefront Data of Normal Eyes. Invest. Ophthalmol. Vis. Sci. 2008;49(13):994. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the potential advantage of a range of image quality metrics (IQMs) over pupil plane metrics in the objective determination of the sphero-cylindrical (SC) refraction.
Data from both eyes of 60 normal subjects were used. Subjective spherical refractive correction ranged from –8.5D to +1.5D and cylindrical correction from –3.75D to 0D. All subjects underwent subjective refraction in mesopic luminance conditions, viewed the 20/20 line of a high contrast Bailey-Lovie acuity chart, and provided subjective feedback to achieve optimal image quality. The same clinician performed all subjective refractions. The objective refraction was assessed from the wavefront aberrations measured with a COAS wavefront sensor. In the pupil plane, estimates based on lower and higher order Zernike polynomials as well as the concept of refractive Zernike power polynomials (Ophthal. Physiol. Opt., 27:245-255, 2007) were used to derive the objective SC correction. In the image plane, a range of IQMs was used including the entropy of the PSF, neural sharpness, visual Strehl ratios based on MTF and OTF, and the augmented VSOTF. To determine the SC correction from the IQMs, an optimization routine was developed that would mimic the method of subjective refraction. First, the initial estimate of the SC correction was removed from the wavefront data. This was followed by the addition of a range of SC corrections to the residual wavefront and calculation of the IQMs. The resultant SC estimate that corresponded to the best IQM outcome was used to adjust the initial estimate of the SC refraction.
The SC refraction was represented in vector components M, J0, and J45. For the pupil plane based metrics, the best result has been achieved with the refractive Zernike power polynomials resulting in r2 values of 0.97, 0.85, and 0.81 for M, J0, and J45, respectively. Correspondingly, PSF entropy achieved r2 values of 0.97, 0.82, and 0.61; neural sharpness 0.97, 0.82, and 0.59; VSMTF 0.97, 0.78, and 0.53; VSOTF 0.97, 0.73, and 0.51; and augmented VSOTF 0.97, 0.78, and 0.58.
Despite having taken account of pupil appodization and neural weighting vision quality metrics derived in the image plane do not provide any substantial advantages over simple pupil plane metrics in the determination of the objective SC refraction for normal eyes. The recently introduced concept of refractive Zernike power polynomials that represent the wavefront information in the refractive power domain provides the best correlation with the subjectively evaluated refraction.
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